1. Field of the Invention
Embodiments herein present a complementary metal oxide semiconductor (CMOS) with dual metal gates.
2. Description of the Related Art
In current metal oxide semiconductor field effect transistors (MOSFETs), a polysilicon gate and an oxide dielectric are typically employed. To enhance device performance and reduce manufacturing costs, critical dimension, i.e., gate length, is aggressively scaled down. This requires effective thickness of gate dielectric to be scaled down to obtain proper device properties such as good control of short channel effect. Due to polysilicon depletion, the polysilicon gate can increase the effective thickness by about 0.5 nm, which contributes 25% of the effective thickness for current high performance MOSFETs. Thinnest gate oxide is about 1 nm in current technology, which contributes 50% of the effective thickness. Another 25% of the effective thickness comes from the channel of a MOSFET. Since gate leakage current increase dramatically when the gate oxide thickness is thinner than 1 nm and then it causes the increasing of power consumption, it is difficult to decrease the oxide thickness further. Therefore, it is preferable to replace polysilicon gate by metal gate to reduce the effective thickness of gate dielectric.
MOSFETs including a gate stack comprising a bottom polysilicon portion and a top silicide portion are also known. The layer of silicide in such a gate stack contributes to a decrease in the resistance of the gate. The decrease in resistance causes a decrease in the time propagation delay RC of the gate. Although a silicide top gate region may help decrease the resistance of the transistor, charge is still depleted in the vicinity of the interface formed between the bottom polysilicon gate and gate dielectric, thereby causing a larger effective dielectric thickness or a smaller effective gate capacitance.
Another type of MOSFET that is available is one where the gate electrode is made entirely of a metal. In such MOSFETs, the metal of the gate prevents depletion of charge through the gate. This prevents the increase in effective thickness of the gate capacitor and the capacitance decreases as a result of the depletion effect.
In order to obtain desirable threshold voltages for nMOSFET and pMOSFET with metal gates, usually different type of metals are needed. It is generally quite difficult to offer metal gate transistors with different type of metals within the same wafer. Thus, semiconductors having metal gate transistors with different type of metals on the same wafer are needed in order to provide design flexibility for low-power, high-performance, and zero mixed-signal applications for overall system performance.